The utilization of implanted hearing instruments continues to increase. In this regard, implantable hearing devices provide operative and cosmetic advantages relative to conventional ear canal hearing devices. For example, implantable hearing devices offer operative advantages in relation to patient's having certain types of conductive or sensorineural hearing loss (e.g. mixed hearing loss comprising a conductive loss component of 45 dB or more with sensorineural hearing loss component of 40 dB or more). These patients are generally known to perform poorly with conventional hearing aids because their conductive and sensorineural hearing loss components are additive and these patients require substantial amounts of gain and output for proper speech recognition.
Typically, an implanted hearing instrument may comprise implanted componentry for stimulating a middle ear component of a patient's auditory system, or alternatively, for electrically stimulating a component of a patient's auditory system. In the former regard, one type of middle ear stimulation device includes an electromechanical transducer having a magnetic coil that drives a supported vibratory actuator positioned to contact and mechanically stimulate the ossicular chain of a patient. In another approach, a magnet is attached to the ossicular chain of a patient and a spaced coil is energized to generate a fluctuating magnetic field to induce magnet movement at acoustic frequencies.
In relation to electrical stimulation approaches, or auditory neurostimulation, known devices include auditory brain stem implant (ABI) devices and cochlear implant (CI) devices. In the case of CI devices an electrode array is inserted into the cochlea of a patient, e.g. typically into the scala tympani so as to access and follow the spiral currature of the cochlea. The array electrodes are selectively driven to stimulate the patient's auditory nerve endings to generate sound sensation. In this regard, a CI electrode array works by utilizing the tonotopic organization, or frequency-to-location mapping, of the basilar membrane of the inner ear. In a normal ear, sound vibrations in the air are transduced to physical vibrations of the basilar membrane inside the cochlea. High frequency sounds do not travel very far along the membrane, while lower frequency sounds pass further along. The movement of hair cells, located along the basilar membrane, creates an electrical disturbance, or potential, that can be picked up by auditory nerve endings that generate electrical action pulses that travel along the auditory nerve to the brainstem. In turn, the brain is able to interpret the nerve activity to determine which area of the basilar membrane is resonating, and therefore what sound frequency is being sensed. By directing which electrodes of a CI electrode array are activated, cochlear implants can selectively stimulate different parts of the cochlea and thereby convey different acoustic frequencies corresponding with a given audio input signal.
With ABI systems a plurality of electrodes may be implanted at a location that bypasses the cochlea. More particularly, an array of electrodes may be implanted at the cochlea nucleus, or auditory cortex, at the base of the brain to directly stimulate the brainstem of a patient. Again, the electrode array may be driven in relation to the tonotopic organization of a recipient's auditory cortex to obtain the desired sound sensation.
As may be appreciated, in the case of either middle ear stimulation devices or neurostimulation devices, audio signals from a microphone may be processed, typically utilizing what is referred to as a speech processor, to generate signals to drive the stimulation device. In this regard, as implanted hearing instruments have continued to evolve, the utilization of implanted microphones has increased. However, the employment of implanted microphones has presented a number of challenges in relation to realizing a desired signal-to-noise ratio with adequate sensitivity across a normal hearing range of acoustic frequencies.